Page 115 - Materials Chemistry, Second Edition

96 Life Cycle Assessment of Wastewater Treatment

Regarding solidity properties, carbonyl dyes are often superior to their azo coun-
terparts, which justifies their selection when a high technical behavior is required
for a particular application. A particular textile application dominated by carbonyl
dyes is the use of vat dyes, a group of dyes applied to cellulosic fibers such as cotton.

6.1.2.3 Phthalocyanines
Phthalocyanines are undoubtedly the most important chromophore system devel-
oped during the twentieth century. Their fortuitous discovery in 1927 from the
reaction of phthalic anhydride with ammonia in a glass lined reactor enabled the
isolation of an insoluble, dark blue crystalline substance, which turned out to be
iron phthalocyanate (FePc; De Diesbach and Von der Weid, 1927). Although tex-
tile dyes based on phthalocyanines are of limited importance, this class of dyes
provides by far the most important blue and green organic pigments and exhibits
exceptional stability. The absorption band of the metal phthalocyanates depends on
the nature of the central metal ion, the arrangement of the substituents on the outer
rings, and the degree of condensation of the rings. The most widely investigated are
the metal complexes of the first transition series: iron, cobalt, nickel, copper, and
zinc (Demirbaş et al., 2017).

6.1.2.4 Polyene and Polymethine Dyes
Two families of structurally related dyes containing one or more methine groups
(-CH=) belong to this group. Polyene dyes consist of a series of conjugated double
bonds, usually terminating in aliphatic or alicyclic groups, which provides its color.
The best-known group of polyene dyes is formed by carotenoids. Polymethine dyes
are capable of providing a broad range of bright and intense colors but in general,
tend to show relatively low strength properties. This feature has limited their use in
fabrics, where they are mainly restricted to some dispersed dyes for polyesters, cat-
ionic dyes for acrylic fibers, and basic dyes (Langhals, 2004).

6.1.2.5 Aryl Carbonium Ion Dyes
Historically, aryl carbonium ion dyes were the first synthetic dyes developed for tex-
tile applications. In fact, the first commercial synthetic dye, Mauvein, belongs to this
group (Hübner, 2006). Although their use has declined considerably, many of them
still have some importance, especially for their use as basic (cationic) dyes for color-
ing acrylic fibers and paper. Aryl carbonium ion dyes are characterized by the cen-
tral carbon atom being attached to either two or three aromatic rings. Structurally,
they are closely related to polymethinic dyes, especially to cyanine-type dyes, and
provide intense and vivid colors but with lower technical properties compared with
the azo chemical families and phthalocyanines (Gordon and Gregory, 1987).

6.1.2.6 Sulfur Dyes
These are known to be complex mixtures of polymeric molecular species that con-
tain a high proportion of sulfur in the form of sulfur (-S-), disulfide (-S-S-), and
poly-sulfide (-Sn-) bonds, and heterocyclic rings. In general, they are highly insol-
uble in water and are applied to cellulosic fibers. The most outstanding advantage
of sulfurized dyes is that they provide reasonable technical performance at low cost